Miniaturization of electronic devices in recent years increases the demand for secondary batteries with high capacities. Attention is being given to nonaqueous-solvent lithium secondary batteries, which have higher energy densities compared to nickel-cadmium and nickel-hydrogen batteries. Although a lot of studies have already been made to increase battery capacity, demand for advanced battery characteristics urges further increase in battery capacity.
Materials that have been examined for their applications to negative-electrode materials for lithium secondary batteries include amorphous carbons, artificial graphites, and natural graphites.
The need for further increase in battery capacity involves establishing a simple, inexpensive method of producing a material that requires only a small magnitude of pressing load to be applied during electrode formation and that can be used in high electrode densities.
In contrast to the artificial graphites, natural graphites have been widely used on the grounds that they can exhibit large discharge capacities close to the theoretical capacity due to their well-developed graphite crystal structures, that they require only a small magnitude of pressing load to be applied during electrode formation, and that they are inexpensive. Having excellent properties as mentioned above, however, natural graphites still require further studies for the purpose of establishing a simple, inexpensive method of producing a negative-electrode material that can be used in high electrode densities in view of increasing battery capacity.
Under the circumstances, Patent Document 1 discloses that a highly-crystalline natural graphite is subjected to purification treatment in an atmosphere of nitrogen gas or argon gas at a temperature of 2400° C. or higher to thereby produce a natural graphite negative-electrode material with excellent cycle characteristics and storage characteristics.
Patent Document 2 discloses that a natural graphite or artificial graphite having high crystallinity is subjected to mechanical-energy treatment to be made into roundish particles whose crushed surfaces are relatively isotropic, thereby producing an electrode that secures high packing efficiency, has large capacity, and is excellent in load characteristics and cycle characteristics. The document also discloses that when the product of the mechanical-energy treatment exhibits a true density of lower than 2.25 g/cm3 and has insufficient crystallinity, the product is then heat-treated at 2000° C. or higher so as to raise its crystallinity.
Patent Document 3 discloses that a natural graphite having an average diameter within a predetermined range is subjected to reheat treatment at a temperature of 2000° C. or higher such that the reheat-treated product exhibits a Raman R value and a peak half-value width each within a predetermined range in accordance with Raman spectroscopy employing argon-ion laser light, thereby producing a negative-electrode material with excellent load characteristics.
[Patent Document 1] Japanese Patent No. 3188032
[Patent Document 2] Japanese Patent Application Laid-Open Publication No. 2000-223120
[Patent Document 3] Japanese Patent Application Laid-Open Publication No. H11-25979